Unit pavement systems are made up of a plurality of paver units laid on a granular material, e.g., sand or stone aggregate. The paver units are made individually or in combination from a variety of materials such as concrete, clay, natural stone, asphalt, or synthetic materials. An “L-type” edge restraint system is the most popular edge restraint system used in the horizontal surface installation of unit pavement systems. The responsibility of the edge restraint system is to prevent the paver units from horizontal movement over time.
The L-type edge restraint is primarily anchored by nails, spikes, stakes or screws driven through fastener openings provided in a horizontal section, also referred to as the base flange, of the edge restraint and then into the granular sub-base below. The sub-base upon which the unit pavement system and the L-type edge restraint is installed typically extends beyond the edge of the unit pavement system a distance equal to the installed base depth. This provides for load transfer, edge restraint stability, and a base structure to hold the anchoring fasteners in place. There are typically two different types of installation techniques used when installing L-type edge restraints.
The first and most common installation method is referred to as exterior base flange placement. In this method, the base flange, or horizontal section, is installed on the outside of the edge of the unit pavement surface. Ten-inch steel spikes are then driven through fastener openings in the base flange and into the sub-grade below. The number of spikes needed varies depending on the load of the unit pavement system after installation.
The exterior base flange placement installation method is most preferred by installers because it allows the installation of the unit pavement system to be completed prior to edge restraint placement. This installation method also allows for the popular “rip cut” that is typically used by installers for time savings. A “rip cut” is defined as a cutting process used by the installer whereby the paver units are overlaid past a finished line, marked, and then cut along the finished line using a portable hand saw to form a finished edge of the unit pavement system. By not having the edge restraint in place, the paver units do not have a specific tolerance to meet along the perimeter of the unit paving system. This process requires less time in cutting labor and allows for a variance in the finished pavement edges or perimeters that is not available with an interior flange placement method that is to be discussed below.
When the “L-type” edge restraint is installed using the exterior base flange placement method, the edge restraint typically moves from its original position over time. The edge restraint is held in place by the vertical force on the spike as the spike is engaged with the sub-base and the weight of any backfill material on the base flange. Snow, rain, ground movement, traffic load, erosion, frost and terrestrial sub-base composition force the edge restraint and the spike to move both vertically and horizontally out of place. This can result in the spike and the edge restraint separating from the sub-base. In other cases, the spike can separate from the edge restraint. When either case occurs, the final result is a failure of the “L-type” edge restraint to stay in place making the edge restraint less effective in holding the paver units in place.
The second installation method for use with the “L-type” edge restraint is referred to as the interior base flange placement method. In this method, the base flange is installed facing toward the finished edge of the unit paving system. This installation is least preferred by installers because the edge restraint needs to be installed and anchored prior to the finished edge of the unit pavement system being installed. In this installation, the paver units must be individually cut and then placed in between the installed paver units and next to the vertical section of the already installed edge restraint. This method is much more time consuming than the exterior base flange placement method. The interior base flange placement method requires individually cutting paver units, precise measurement of edging placement, and results in additional labor time to fix units that do not line up with the edge restraint during installation.
With the interior base flange placement installation method, the weight of the paver units along the edge of the unit pavement system can help restrain the edge restraint from movement. One disadvantage with this method, however, is that the outer edges of the paver units along the edge of the unit pavement system sit on a different foundation structure, i.e., the base flange of the edge restraint, as compared to the remainder of the paver units along the edge, which sit on granular material. This causes the paver units along the edge to tip toward the interior of the unit pavement system. This tipping direction can cause water drainage issues because the free flow of water away from the unit pavement edge is impeded.
There is also another type of edge restraint generally referred to as a permeable pavement edge restraint. Permeable pavement edge restraints are primarily used for unit pavement systems that have a base composition made of larger aggregate materials having no small or fine particles. Such a base composition does not compact as densely as traditional aggregate. Since the material is open-graded and not finely compacted, the use of common fasteners, e.g., the 10-inch spike mentioned above, is unavailable because the common fasteners will not stay in place as they would in a traditional finer aggregate base. A geo-grid is typically installed on the sub-base prior to the unit pavers being installed. The permeable pavement edge restraints are then fastened to a capture plate or clip that holds the edge restraint to the geo-grid or base plate. Generally, this system requires additional labor and materials and is more costly than a typical L-type edge restraint system.